WASHINGTON (AP) — A Republican senator with access to some of the nation's top secrets became further entangled in a deepening FBI investigation as agents examining a well-timed sale of stocks during the coronavirus outbreak showed up at his home with a warrant to search his cellphone.

Hours later, Sen. Richard Burr of North Carolina stepped aside Thursday as chairman of the powerful Senate Intelligence Committee, calling it the “best thing to do." Burr has denied wrongdoing.

“This is a distraction to the hard work of the committee and the members, and I think that the security of the country is too important to have a distraction,” Burr said. He said he would serve out the remainder of his term, which ends in 2023. He is not running for reelection.

The search warrant marked a dramatic escalation in the Justice Department’s investigation into whether Burr exploited advance information when he unloaded as much as $1.7 million in stocks in the days before the coronavirus caused markets to plummet. Such warrants require investigators to establish to a judge that probable cause exists to believe a crime has occurred.

The warrant was confirmed by two people familiar with the matter, including a senior department official. They spoke on condition of anonymity because they were not authorized to publicly discuss an ongoing investigation.

Burr faces no public accusations by the government that he exploited inside information received during briefings. But the search warrant immediately affected the standing inside Congress of the influential Republican, who has earned bipartisan support for leading a congressional investigation into Russian interference in the 2016 presidential campaign — work that sometimes rankled President Donald Trump and his supporters.

News of the warrant also underscored the public scrutiny surrounding the stock market activities of multiple senators and their families around the same time.

On Thursday, a spokesman for Sen. Dianne Feinstein, D-Calif., said she was asked “some basic questions" by law enforcement about sales her husband made and had voluntarily answered questions.

Republican Sen. Kelly Loeffler, a new lawmaker from Georgia, and her husband dumped substantial portions of their portfolio and purchased new stocks around the time Congress was receiving briefings on the seriousness of the pandemic. Loeffler has said she had no involvement in the trades and said they were managed by third-party advisers.

A spokesperson said Loeffler has forwarded documents to the Justice Department, the Securities and Exchange Commission and the Senate Ethics Committee “establishing that she and her husband acted entirely appropriately and observed both the letter and the spirit of the law.”

In Burr's case, the search warrant was served on a lawyer for him, and FBI agents went to the senator's home in the Washington area to retrieve the cellphone, the Justice Department official said. The decision to obtain the warrant was approved at the highest levels of the department, the official said.

Alice Fisher, a lawyer for Burr, noted that Burr called for an ethics inquiry into the stock sales once they were disclosed. She said the senator has “been actively cooperating with the government’s inquiry, as he said he would."

“From the outset, Senator Burr has been focused on an appropriate and thorough review of the facts in this matter, which will establish that his actions were appropriate,” Fisher said in a statement.

Trump, speaking to reporters at the White House before traveling to Pennsylvania on Thursday, said he was unaware that Burr was leaving his intelligence post.

“I know nothing about it — never discussed it with anybody,” Trump said. “That’s too bad.”

Senate records show Burr and his wife sold between roughly $600,000 and $1.7 million in more than 30 transactions in late January and mid-February, just before the market began to dive and government health officials began to sound alarms about the virus. Several of the stocks were in companies that own hotels.

Burr has acknowledged selling the stocks because of the coronavirus but said he relied “solely on public news reports,” specifically CNBC’s daily health and science reporting out of Asia, to make the financial decisions.

Senators did receive a closed-door briefing on the virus on Jan. 24, which was public knowledge. A separate briefing was held Feb. 12 by the Senate Health, Education, Labor and Pensions Committee, of which Burr is a member. It’s unclear if he attended either session.

He was first elected to the Senate in 2004 and chaired the Senate Intelligence Committee as it conducted its own investigation into Russian election interference in the 2016 presidential election. The committee last month issued a report supporting the conclusion by U.S. intelligence agencies that Russia had interfered on Trump’s behalf.

As chairman, Burr has access to the most sensitive classified information that is provided to Congress. Along with Republican and Democratic leadership and the top lawmakers on the House Intelligence Committee, Burr is part of the group of eight lawmakers in Congress who receive the highest level classified briefings.

His decision to leave the chairman position surprised fellow committee members.

Sen. John Cornyn, R-Texas, said he respected Burr’s decision, adding that “he's entitled to a presumption of innocence just like anybody else.”

"The best I can tell, he is trying to do the right thing by the Senate, and I appreciate it,” Cornyn said.

It’s unclear who will take Burr’s place. The next several Republican members in seniority are already chairmen of other committees, though they could choose to switch.

Next in seniority is Idaho Sen. James Risch, who told reporters on Thursday that he didn’t know whether he would keep his current perch as chairman of the Senate Foreign Relations Committee or move to the intelligence panel.

Following him is Florida Sen. Marco Rubio, who heads the Senate Small Business Committee. He said that he wasn’t aware that Burr was stepping aside and that the decision on who takes over was up to Senate Majority Leader Mitch McConnell, R-Ky.

Maine Sen. Susan Collins, chairwoman of the Senate Aging Committee, is third in line.